Variable optical attenuator

ABSTRACT

An optical attenuator ( 10 ) includes: an optical splitter ( 11 ), a collimator ( 12 ), two detectors ( 51, 52 ), a first and second reflectors ( 21, 22 ), an attenuating element ( 3 ) and a driving device ( 4 ). The optical splitter includes a ferrule ( 112 ) and a GRIN (graded index) lens ( 113 ). The collimator is similar to the optical splitter. Input optical signals are transmitted from an input fiber ( 110 ) through the optical splitter and are then directed to the first reflector. The optical signals reflected by the first reflector pass through the attenuating element and are subsequently reflected to the collimator by the second reflector. The two detectors receive sampling signals via an input and an output sampling fibers ( 111, 112 ). The driving device can drive the attenuating element in response to the attenuation ratio coming from the two detectors.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical attenuator, andparticularly to a variable optical attenuator having an electricalcontrolling circuit.

[0003] 2. Description of the Related Art

[0004] Optical attenuators are used to optimize the optical power ofsignals at key points in optical communications networks. For example,in networks having Erbium Doped Fiber Amplifiers (EDFAs), opticalattenuators are used between stages of EDFAs to provide constant gain.In Wavelength Division Multiplexer (WDM) systems, optical attenuatorsare used to adjust optical power of “added” laser signals to match thesignals strength of other channels within the network. Opticalattenuators can also be used to set signal strength within the range ofa particular receiver.

[0005] Known methods to obtain a variable optical attenuator includecoating a filter element with an attenuation layer having a variabledensity, and bending optical fibers to get a given attenuation. Avariable optical attenuator can also be obtained by changing a distancebetween a reflector and an input port or an output port.

[0006] U.S. Pat. No. 5,745,634 discloses a voltage controlled attenuatorcomprising a first lens for receiving incoming optical signals, a secondlens for outputting the attenuated optical signals, an optical powerdetector and a controllable attenuating element. The optical powerdetector monitors the intensity of the attenuated optical signals, andthe controllable attenuating element varies the attenuation of theoutputting optical signals in response to electrical signals from theoptical power detector. A weakness of this prior art arrangement is thatthe optical power detector is separate from the second lens. Theattenuated optical signals reflected by an input face of the second lenshave to travel a distance to the optical power detector, which wastes aportion of the reflected optical signals. An arrangement which utilizesthe reflected signals more efficiently is desired.

BRIEF DESCRIPTION OF THE INVENTION

[0007] An object of the present invention is to provide an opticalattenuator which controllably attenuates optical signals by using anelectrical controlling circuit.

[0008] Another object of the present invention is to provide an opticalattenuator which accurately and flexibly controls attenuation of opticalsignals by using an optical splitter and two sampling detectors.

[0009] An optical attenuator in accordance with the present inventioncomprises: an optical splitter, a collimator, an input and an outputdetectors, a first and a second reflectors, an attenuating element and adriving device. The splitter includes a ferrule and a GRIN (gradedindex) lens. Input optical signals are transmitted from an input fiberthrough the splitter and are then directed to the first reflector. Theoptical signals reflected by the first reflector pass through theattenuation element and subsequently are reflected to a collimator bythe second reflector, and are then directed to an output fiber. Theinput and output detectors detect the intensity of input optical signalsand output optical signals and a control circuitry calculates theattenuation ratio. The driving device then drives the attenuatingelement in response to the attenuation ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a schematic diagram of a variable optical attenuatoraccording to the present invention.

[0011]FIG. 2 is a schematic diagram view of a splitter of the variableoptical attenuator according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0012] As shown in FIG. 1, an optical attenuator 10 of the presentinvention comprises: an optical splitter (i.e., collimator) 11, acollimator 12, an input and an output detectors 51, 52, a first and asecond reflectors 21, 22, an attenuating element 3 and a driving device4. The optical attenuator 10 attenuates optical signals traveling alongan optical path.

[0013] Referring to FIG. 2, the optical splitter 11 includes a ferrule112 and a GRIN (graded index) lens 113. The ferrule 112 is closelyattached to the GRIN lens 113. One end of each of an input fiber 110 andan input sampling fiber 111 are retained in the ferrule 112. The GRINlens 113 has a first surface 114 attached to the ferrule 112 and asecond surface 115 opposite to the first surface 114. An antireflectionfilm is coated on the first surface 114 and a beam splitter film iscoated on the second surface 115. Little reflection of the opticalsignals will occur at the antireflection film, but a portion of theoptical signals will be reflected at the beam splitter film.

[0014] The collimator 12 is similar to the optical splitter 11 and alsocomprises a ferrule (not labelled) and a GRIN (graded index) lens (notlabelled). An output fiber 120 and an output sampling fiber 121 areretained in the ferrule of the collimator 12. However, both end surfacesof the collimator's GRIN lens are just coated with an antireflectionfilm, and optical signals transmitted from the second reflector 22 tothe collimator 12 are automatically split into two portions,respectively received by the output fiber 120 and the output samplingfiber 121.

[0015] The input and output detectors 51, 52 are respectively connectedto the input sampling fiber 111 and the output sampling fiber 121 fordetecting the intensity of optical signals transmitted through thesampling fibers 111, 112. The input and output detectors 51, 52 can bephotodiodes.

[0016] The first and second reflectors 21, 22 are arranged at such anangle that the first reflector 21 ensures that most of the opticalsignals transmitted from the optical splitter 11 are directed to theattenuating element 3 and the second reflector 22 ensures that most ofthe attenuated optical signals transmitted from the attenuating element3 are received by the collimator 12.

[0017] The attenuating element 3 is used for attenuating optical signalspassing through it. A conventional variable neutral density filter or aconventional wedge shaped filter can be used here. The attenuatingelement 3 is attached to the driving device 4. A linear movement of theattenuating element 3 in a direction perpendicular to the path ofoptical signals passing through it can lead to a graduated change ofattenuation of the signals. Furthermore, the driving device 4 can drivethe attenuating element 3 in response to an attenuation ratio outputfrom a control circuitry (not shown).

[0018] In use, input optical signals are transmitted through the inputfiber 110 into the optical splitter 11, and then pass through the GRINlens 113 to the first reflector 21. The beam splitter film coated on thesecond surface 115 of the GRIN lens 113 reflects a small part of theinput optical signals back into the input sampling fiber 111. In thisembodiment, 5 percent of the input optical signals are reflected anddirected through the input sampling fiber 111 to the input detector 51.The other 95 percent of the input optical signals are transmittedthrough the optical splitter 11 and are reflected by the first reflector21 to travel through the attenuating element 3. The attenuated opticalsignals from the attenuating element 3 are reflected by the secondreflector 22 and are received in the collimator 12. About 5 percent ofthe attenuated optical signals are split and directed to the outputdetector 52 through the output sampling fiber 121. The remaining 95percent if the attenuated optical signals are directed as output opticalsignals to the output fiber 120. After the intensities of the opticalsignals are detected by the input and output detectors 51, 52, a controlcircuitry (not shown) is used to calculate the intensity of the inputoptical signals and the output optical signals. The control circuitryalso calculates an attenuation ratio from the signals received from thedetectors 51,52, and the driving device 4 then drives the attenuatingelement 3 to move to an appropriate position to achieve a desiredattenuation ratio.

[0019] The advantage of the system of the present invention is that itprovides greater efficiency in controlling the desired attenuation ofthe signals. Instead of directly reflecting a beam of light to thedetector as in the prior art, the present invention transports the smallpart of the input optical signals transmitted to the input detector 51via a fiber optic link—the input sampling fiber 111. The optical signalsent to the input detector 51 therefore is more secure and efficient.

[0020] It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size, and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meaning of the terms inwhich the appended claims are expressed.

1. An optical attenuator for attenuating optical signals in an opticalpath, comprising: an optical splitter for splitting an input opticalsignals from an input fiber into two portions, one portion of the inputoptical signals being transmitted to an attenuating element and a secondportion being transmitted to a first detector; an output port forsplitting attenuated optical signals into two portions, one portion ofthe attenuated optical signals being transmitted to an output fiber anda second portion being transmitted to a second detector; and anattenuating element for attenuating the input optical signals, theattenuating element being driven by a drive device in response tosignals from the first detector and the second detector; wherein thefirst detector is positioned to receive said second portion of the inputoptical signals and the second detector is positioned to receive saidsecond portion of the attenuated optical signals.
 2. The opticalattenuator as claimed in claim 1, wherein the optical splitter comprisesa ferrule and a GRIN lens.
 3. The optical attenuator as claimed in claim2, wherein the ferrule retains an end of the input fiber and an end of asecond fiber, and said second fiber receives the second portion of theinput optical signals.
 4. The optical attenuator as claimed in claim 1,wherein the output port is a collimator.
 5. The optical attenuator asclaimed in claim 2, wherein the GRIN lens has a first surface coatedwith an antireflection film and a second surface coated with a beamsplitter film.
 6. The optical attenuator as claimed in claim 1, whereinthe first detector and the second detector respectively include aphotodiode.
 7. An optical attenuator for attenuating signals in anoptical path comprising: an optical splitter including a ferrule and aGRIN lens for splitting an input optical signals from an input fiberinto two portions, one portion of the input optical signals beingtransmitted to an attenuating element and a second portion beingtransmitted to a first detector; an output port for splitting attenuatedoptical signals into two portions, one portion of the attenuated opticalsignals being transmitted to an output fiber and a second portion beingtransmitted to a second detector; and an attenuating element forattenuating the input optical signals, the attenuating element beingdriven by a drive device in response to signals form the first detectorand the second detector; wherein the first detector is positioned toreceive said second portion of the input optical signals and the seconddetector is positioned to receive said second portion of the attenuatedoptical signals.
 8. The optical attenuator as claimed in claim 7,wherein the ferrule retains an end of the input fiber and an end of asecond fiber, and said second fiber receives the second portion of theinput optical signals.
 9. The optical attenuator as claimed in claim 7,wherein the output port is a collimator.
 10. The optical attenuator asclaimed in claim 7, wherein the GRIN lens has a first surface coatedwith an antireflection film and a second surface coated with a beamsplitter film.
 11. The optical attenuator as claimed in claim 7, whereinthe first detector and the second detector respectively include aphotodiode.
 12. An optical attenuator comprising: an input collimatorincluding an input collimator with a first GRIN lens and a firstferrule, main and sample input fibers retained in the first ferrule, abeam splitter film applied on the said first GRIN lens; an outputcollimator including an output collimator with a second GRIN lens and asecond ferrule, man and sample output fibers retained in the secondferrule, a beam splitter film applied on said second GRIN lens; an inputdetector connected to a distal end of said sample input fiber; an outputdetector connected to a distal end of said sample output fiber; and anattenuation element interrupting a light path defined between said inputGRIN lens and said output GRIN lens; wherein said attenuation element iscontrollable to move according to detection results from both said firstand second detectors.